Re: [PATCH v3 4/20] PCI/P2PDMA: introduce helpers for dma_map_sg implementations

[Logan Gunthorpe]

On 2021-09-28 4:05 p.m., Jason Gunthorpe wrote:
> On Thu, Sep 16, 2021 at 05:40:44PM -0600, Logan Gunthorpe wrote:
> 
>> +enum pci_p2pdma_map_type
>> +pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, struct device *dev,
>> +		       struct scatterlist *sg)
>> +{
>> +	if (state->pgmap != sg_page(sg)->pgmap) {
>> +		state->pgmap = sg_page(sg)->pgmap;
> 
> This has built into it an assumption that every page in the sg element
> has the same pgmap, but AFAIK nothing enforces this rule? There is no
> requirement that the HW has pfn gaps between the pgmaps linux decides
> to create over it.

No, that's not a correct reading of the code. Every time there is a new
pagemap, this code calculates the mapping type and bus offset. If a page
comes along with a different page map,f it recalculates. This just
reduces the overhead so that the calculation is done only every time a
page with a different pgmap comes along and not doing it for every
single page.

> At least sg_alloc_append_table_from_pages() and probably something in
> the block world should be updated to not combine struct pages with
> different pgmaps, and this should be documented in scatterlist.*
> someplace.

There's no sane place to do this check. The code is designed to support
mappings with different pgmaps.

Logan